Pumping system



M/VEWMIP M. B. SENNET PUMPING SYSTEM & m M .5 y a 3% 0 WW m Filed March 11, 1943 Aug. 7, 1945.

Patented Aug. 7, 1945 UNITED STATES PATENT OFFICE" I PUMPING SYSTEM Morgan Sennet, Trenton, ,J.,' assignor .to De Laval Steam Turbine Company, Trenton, N.J.,'a corporation of New Jersey 1 AppllcationMarch 11, 1943, Serial No. 478,752

zoiaims. (c1. lea-i1) This invention relates to a pumping system and has particular relation to a system involving a positive pump, for example of the rotating screw type following the disclosure of the patcuts to C. O. J. Montelius 1,690,802, 1,821,523 and Pumps constructed in accordance with said patents .comprise a plurality of screws which,

during operation, provide theoretically completely bounded chambers travelling endwise' of the screws and adapted to transfer fluid axially as completely sealed. Pumps of this type,therefore, are truly positive displacement pumps capable of delivering liquids at very high pressures.

Pumps of this type, furthermore, may be operated at extremely high speeds with a driving screw connected, for example, directly to a turbine wheel. Operating speeds may run .as high as 24,000 R. P. M. As a result of these very high speeds of operation, pumps of quite small'size may be made with very large capacity.

At the extremely high operating speeds just that an intermittent approach flow of the liquid is involved, since the action is such that the displacement is practically uniformly continuous.)

The high rates of flow with the accompanying production of local turbulence tend to cause the setting up of cavitation with accompanying sub-" stantialliquid pressure drops, with the result that if a liquid such as an oil containing air and volatile constituents is being pumped, these;may separate out in gaseous or vapor. form at the inlet. whatever pressure existed at the inlet is essentially maintained, and the travelling chamber may contain liquid with free vapor or gas. At the discharge, this chamber is again opened, but now to the high pressure of the discharge, whereupon, if there is any free vapor, this is recompressed and probably dissolved or absorbed with resulting noise and vibration which may attain such magnitudes as to damage piping.

It has been proposed to provide a booster stage As soon as the chamber is completed,-

.o'fvibration so long as liquid only is'being handied, irrespective of the pressure" thereon. The use of a centrifugalpump in this fashion, however, is unsatisfactory in several respects. First, theefilciency is low. Secondly,there is no flexibility in the matter of control of the pressure imposedon theintake ofthe? screw pump. Third,

if the amount of air or vapor is large, the. centrifugal pump may lose" its prime, even though the second stage,that isthe screw pump, is of the positive displacement type. Under these conditions; the screw -pump may draw enough volume of mixed liquid and'vapor to satisfy'its volumetric change, but the first stage centrifugal pump will do no'work, and'will fail toicreate the necessary intake pressure. a r I.

In accordance :with the present invention,

there are provided two-screw pumps, preferably of the type indicated'above, one'of whichforms the main pump and is preceded by another which acts asa booster g vI The solution of the problems involved, as: well as other objects of the invention, particularly relating 1 to details, will become apparent. from the following description,- pread ini'conjunction with the accompanying drawing, in ,whichf'the figure shows, in sectioniand diagrammatically,

the improved type of pump, together with associated flow controlling device;

The pump comprises a casing 2 in which there are located interior elements 4 and .6 providing housings within which there rotate thescrews of the two pumps. The first stage comprises a central screw 8, cooperatingwith a pair of screws -l0, oneof which is illustrated in the figure, the

other being cut away by reason of' the section. As will be evident from the Montelius patents, various screw arrangements may be provided to secure the travelling fluid-tight chambers, for example, consisting of two screws, one of which is provided with a single thread and the other of which carries two threads, or three screws each of which is provided with two threads. This latter type of arrangement is usually preferable, and is the one illustrated, because of its balanced nature. In order to secure a comone of the side ones of which is indicated at 4. As will be evident from the figure, there is free fiuid communication between the two pumps in theregion 22, so that liquid entering the right hand first stage through the intake passage [81" and the passages may flow through the two pumps in series to be discharged into the space 24 connected with the delivery passage 26.

In order to secure the boosting effect of the first stage, there are two possible alternative ar-' rangements, or combinations thereof, to secure a greater displacement in the first pump than in the second, so that the first will deliver a greater volume of fluid. In-the first stage, there may, be provided screws of the same pitch as in the second stage, but,of greater diameter. Alternatively, and-preferably, the screws of the two pumps arethesame in diametenbut the screws of the first stagehave a greater pitch than-those of-the secondstage, it being found, for example,

that very=good operation is achieved if, as illustrated, the pitch of the screwso'f the first stage is about25% greater than thepitch of the screws of the second stage. Iithe screws are of the same diameters, the center lines of the rotors are in line and the-design is very much simplified, especially'if rotor end thrust is to be taken care of, as'illustratedin the figure.

The first stage, in view of'the greater pitch of its screwspwill deliver a greater-volume than the second stage, and to effect satisfactory operation, thereis provided a bleed-off from the space "between the two pumps. As illustrated diagrammaticallyat the bottom ofthe figure, a connection to the openingl28 in the casing permits liquid to flow from the space 22 to: an adjustable valve indicated at 32; the control being effected by restriction of-the flow by a'valve member 34,

urged in a direction to close oifthe passage-30 by a spring 36, the pressure exerted by whichmaybe adjustedby movement of an abutment through the medium of a handle 40; From the valve 32, the bled-off liquid passes through the connection 42 to the supply of the pump.

By reason of the provision of the valve just described, the'excess liquid is recirculated, and thereis maintained between the pumps a proper pressure to insure the prevention, of objectionable cavitation, as: mentioned above. The adjustment ismade such as to prevent too great a pressure rise across thefirst stage. Desirably the pressure in the space 22 is kept reasonably low, iorexamplaless than ten pounds per square inch, under which conditions there will be no vibration arising in the first stage. If a moderate pressure of this sort is provided, then vibration in the second stage is also completely avoided despite the fact that the second stage may be operating against a very high pressure. It has been found, for example, that with the conditions just indicated, a mixture of air and oil may be pumped satisfactorily even though the air may amount to 40% of the, volume of the mixture. The, adjustmentprovided ing the valve 32 may be such as to maintain at 22 the necessary pressure for varying contents of vapor or air in the liquid being pumped.

As compared with the combination of a screw pump with a centrifugal first stage, the efficiency ,of the pum described is quite high. Due to the .m-Ovision of an adjustable valve controlling the bleed-off, it is possible to make the proper pres- I sure adjust'mentt'o correspond with the air or vapor content of the pump liquid. There is, furthermore, no question of priming involved,

since thepositive first stage is completely selfpriming.

What I claim a d desire'to protect by Letters Patent is:

I 1. In combination, a positive displacement multiple screw pump, a second positive displacement multiplescrew' pump operating in unison with the first and having at least one screw coaxial with a screw of the first and operating at the same rotary speed, the second pump'havinga greater liquid displacement per unit time than the first pump, said pumps having a common housing and receiving and delivering liquid axially of 'thescrews and the progress of liquid in both pumps being in the same axial direction so that the first pump receives directly the liquid discharged from the second pump, means for directregion of the housing between the pumps, and

first pump, means for-directing liquid from the means forregulating the pressure in the last mentioned means.

'ment multiple screw pump operating in unison 2. In combination, a positive displacement multiple screw pump, a second positive displacethan the first pump, said pumps having a common housing and" receiving and delivering liquid axially of the screws and the progress'of liquid in both pumps being in the sameaxial direction so that the first pump 'receiv'es directly the liquid discharged from the second pump, means for directing liquid to'the inlet of said second pump, means'for directing liquid from the outlet of the 'first pump, means {or directing liquid from the region of the housing between the pumps, and a spring-controlled valve controlling diversion of liquid-through the lastmentioned me'ans'to maintainsubstantiall y constant the pressure existin between the-pumps.

-- ljMORGANESENNET. 

